Various portable electronic measuring instruments are currently available. One example of such an instrument is a displacement measuring instrument, such as a hand-held electronic caliper which can be used for making precise measurements of machined parts to ensure that they are meeting tolerance requirements.
An example of an electronic caliper using highly accurate measuring techniques is shown in U.S. Pat. No. 5,901,458, which is commonly assigned and hereby incorporated by reference in its entirety. As described, the electronic caliper has a reduced offset position transducer that uses a read head that is movable along a scale. The electronics provide a precise measurement of the read head's position on the scale. The transducer uses two sets of coupling loops on the scale to inductively couple a transmitter winding on the read head on a slide to one or more receiver windings on the read head. The transmitter winding generates a primary magnetic field. The transmitter winding is inductively coupled to first loop portions of first and second sets of coupling loops by a magnetic field. Second loop portions of the first and second sets of coupling loops are interleaved and generate secondary magnetic fields. A receiver winding is formed in a periodic pattern of alternating polarity loops and is inductively coupled to the second loop portions of the first and second sets of coupling loops by the secondary magnetic fields. Depending on the relative position between the read head and the scale, each polarity loop of the receiver winding is inductively coupled to a second loop portion of either the first or second set of coupling loops. The relative positions of the first and second loop portions of the first and second sets of coupling loops are periodic and dependent on the relative position of the coupling loops on the scale.
Another example is shown in U.S. Pat. No. 5,886,519, which is commonly assigned, and hereby incorporated by reference in its entirety. The '519 patent discloses an inductive absolute position transducer for high accuracy applications, such as linear or rotary encoders, electronic calipers and the like. The absolute position transducer uses two members movable relative to each other. The first member contains at least one active transmitter for generating a magnetic field and at least one receiver for receiving the generated magnetic field. The passive second member includes passive flux modulating elements that modulate the received field depending on their position relative to the at least one receiver. An electronic circuit coupled to the at least one transmitter and the at least one receiver compares the outputs of the at least one receiver, evaluates the absolute position between the two members, and exhibits the position on a display. The inductive absolute position transducer determines the absolute position between the two members.
Another example is shown in U.S. Pat. No. 5,804,963, which is hereby incorporated by reference in its entirety. FIGS. 3, 4, and 6 of the '963 patent have been reproduced herein as prior art FIGS. 1, 2, and 3, as will be described in more detail below. In summary, the '963 patent discloses an inductive displacement sensor comprising two elements moveable relative to each other along a path, provided with windings arranged along the path and where the inductive coupling between the windings varies as a periodic function of the relative displacement of the two elements, and electronic means for determining the value of the displacement from a measurement of the inductive coupling between the windings. As will be described in more detail below, the '963 caliper uses three identical coils, with one of the coils selected as a primary (transmitter). The other two are selected as secondaries (receivers) and the output is measured differentially. The scale causes spatially periodic variations of the mutual inductance between the coils. The 3 coils are multiplexed and switched or “selected” to be either a primary or a secondary (“rotated”) 3 times to create 3 phases. The selected primary coil is driven by a 25 nS pulse. At the end of the pulse, the selected receiver outputs are sampled and processed. The 3 phase outputs are taken in regular intervals and used to create a “staircase” signal that is filtered, using a phase measurement technique.
As shown in FIG. 1, in the '963 caliper a first element or cursor 31 with N=3 interlaced meander windings 31A, 31B, 31C, of pitch 2T and successively shifted by 2T/3, faces a second element or scale 32, including a conducting tape featuring a row of windows 321 and traverses 322 on a pitch T. It can be seen that the first element's 31 windings 31A, 31B, 31C are implemented on two metal layers so they may cross each other. Passage, or ohmic contact, from one meander layer to the other is done via contacts 310.
As shown in FIG. 2, in an alternate embodiment of the '963 caliper, windings 41A, 41B, 41C, are formed by two meanders whose linear sections running across the x direction are superposed, resulting in a fourfold inductance over the same area. The inducting or induced currents have the same direction in the superposed linear sections, as shown by arrows in winding 41B, enhanced in FIG. 2. As the meanders of each winding go back and forth, i.e., start and finish at the same end, all connections are at one end, minimizing stray inductance and radiation. The windings in FIG. 2 are tapered towards the ends to equalize their mutual inductances and to reduce this finite length configuration's sensitivity to misalignment in linear sensors. In rotary versions, this can also be achieved by distributing the windings over the whole perimeter.
As shown in FIG. 3, in one embodiment of the electronic circuitry for the '963 caliper, three meander windings A, B, C, having different spatial phases, are provided. They are Y-connected, i.e., with one terminal each on a common contact, connected to a positive voltage V+, which may be the circuit's supply voltage. The remaining terminals LA, LB, LC, are respectively connected to driver transistors TA, TB, TC, to surge absorbing diodes DA, DB, DC, and to identically named terminals LA, LB, LC of transmission gates or switching transistors TG. The driver transistors are N-channel MOS enhancement types.
The measuring principle of FIG. 3 is described as follows. Coupling via the scale is measured by generating a voltage pulse on winding A and simultaneously sampling the difference between the induced voltages on the other windings B and C, the inductor and induced windings being thus in quadrature, as will be seen further. This coupling varies in a spatially periodic manner, with a spatial period T, and a periodic sampled voltage sequence can thus be obtained by generating the next pulse on the following winding B and sampling the voltage difference between the next windings C, A, etc. This sequence yields three samples per period, which is sufficient to find the spatial phase of the scale that influences the sampled signals. This phase is stated to vary linearly with the scale displacement as long as spatial harmonics of the coupling characteristic are negligible. This is usually the case, given that even harmonics are weak, that the third harmonic is not sampled and that those of order five and above are strongly attenuated for a sufficient gap, about 0.2 T, between the winding's and the scale's surface. It is stated that spatial harmonics of a meander winding's electromagnetic field decrease exponentially with the gap, a harmonic of order m decreasing by half for a gap of 0.22 T/m. The signal phase may then be computed as a function of the numerical amplitude and polarity values of said three sampled voltages. It is stated that the electronic means described and illustrated in FIG. 3 determine the phase directly by filtering a signal sampled six times per period, low-pass filtering being simpler for more samples per period. The spatial harmonic three is sampled, but can be filtered. But harmonic five, on the contrary, is stated to be rejected on the fundamental by sampling, so it is attenuated before, e.g., by the shape of the meanders and the scale or simply by a sufficient gap. It is stated that in the '963 caliper the absence of windings and connections on the second element, or scale, often quite long, allows high frequency measuring signals, where the winding's impedance is higher than their ohmic resistance, thus improving the sensor's efficiency.
Systems such as those shown in the '458, '519, and '963 patents utilize advanced signal processing techniques to produce displacement measurements. However, in some of these systems, non-simultaneous measurement of the various winding signals can lead to measurement errors during motion, and/or slower measurement cycles. In some of these systems, simultaneous measurement of the various winding signals may be achieved, but may require additional size, or greater fabrication complexity and cost, in order to provide dedicated “transmitter” windings. In some of these systems, measurement signals and or signal to noise ratio may be low. A variable inductance position transducer that can overcome some or all of these problems would be desirable.